In 1995, the Intergovernmental Panel on Climate Change (IPCC), made up of 2400 scientists, concluded tentatively that "the balance of evidence suggests that there is a discernible human influence on global climate."  Although the specific consequences of human activity are unknown, humans are definitely contributing to a change in atmospheric concentrations by producing gases which intensify the greenhouse effect.  Greenhouse gases include carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons.

CO2
The IPCC estimates that rising CO2 emissions account for about 60% of the warming observed since 1850.  Ice core records have revealed pre-Industrial (pre-1750) global atmospheric concentrations of CO2 of 280ppmv (parts per million volume).  Current measurements show CO2 at 365ppmv.  Sources of CO2 include:

• burning of fossil fuels
• (changing land use) destruction of forests
• biomass burning
• cement manufacture

CH₄

Atmospheric methane is second only to CO2 as an anthropogenic source of the greenhouse effect.  Methane's overall contribution to global warming is large because it is 22 times more effective at trapping heat in the atmosphere than CO2.  Furthermore, methane's concentration in the atmosphere has more than doubled in the last two centuries due mainly to increasing emissions from human sources.
 

NO_x

The concentration of Nitrous Oxides in the atmosphere has also increased slightly since the Industrial Revolution.

CFCs

Chlorofluorocarbons (CFCs) are a group of man-made compounds containing chlorine, fluorine, and carbon.  The manufacture of CFCs began in the 1930s for use in refrigeration units, as cleaning solvents, and in the production of insulating foams.  These compounds are also responsible for the Antarctic ozone hole and a more general decline in global stratospheric ozone, but this is a separate problem from global warming.  CFCs are being phased out under the terms of the Montreal Protocol signed in 1987.

Ozone

Ozone is an important greenhouse gas present in both the stratosphere and troposphere.  Changes in ozone cause radiative forcing by influencing both solar and terrestrial radiation.

Summary

The chart below gives the present concentration of greenhouse gases, the pre-Industrial concentration, the rate of increase in the concentration of gases each year, the residence time (or the amount of time they persist in the atmosphere), the heat absorption capacity (standardized to CO2), and the percentage the gases are thought to contribute to global warming.

Anthropogenic - of, relating to, or resulting from the influence of human beings on nature

Biomass - plant materials and animal waste used especially as a source of fuel

Denitrification - reduction of nitrates or nitrites, commonly by bacteria, that usually results in the escape of nitrogen into the air

Enteric fermentation - a digestion process by which carbohydrates are broken down by microorganisms into simple
                                  molecules for absorption into the bloodstream

Radiative forcing - a change in the energy balance of the radiation entering and leaving the atmosphere

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